echo has 8 distinct synonyms spread across 4 of its 9 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the repetition of a sound resulting from reflection of the sound waves
“she could hear echoes of her own footsteps”
(Greek mythology) a nymph who was spurned by Narcissus and pined away until only her voice remained
a reflected television or radio or radar beam
a close parallel of a feeling, idea, style, etc.
“his contention contains more than an echo of Rousseau”
an imitation or repetition
“the flower arrangement was created as an echo of a client's still life”
Synonyms, antonyms and the broader and narrower relations for echo come from Princeton University’s WordNet. Full sources and licences.